This invention relates to atomizers, and more specifically, to atomizers for atomizing high-viscosity fluids into monodisperse droplets.
Atomization of high-viscosity fluids, fluids having a viscosity much greater than the viscosity of water (.mu..sub.water =1.0 cP), is a fundamental process step in many industrial processes including, for example, food processing, metal powder production, and polymer production. For example, in the plastics industry, polymer solutions are atomized to enhance the evaporation of a solvent during a precipitation phase of plastic powder production. As the amount of polymer increases in proportion to the amount of solvent, the polymer solution becomes more viscous. As the viscosity of a fluid increases, viscous forces that resist droplet formation increase, correspondingly increasing the energy required to atomize the fluid.
Typical industrial practice is to use high-shear, air-blast atomizers that utilize kinetic energy of a carrier fluid, for example, air accelerated to high speed for atomization. Such atomizers require large volumes of high-pressure carrier gas and inherently generate polydisperse droplet size distributions that are generally less desirable than monodisperse droplet size distributions. Control of this type of atomization process is also difficult since droplet formation is dependent on the local conditions of the streams of carrier gas and fluid to be atomized, for example, turbulence levels, momentum fluxes, and mixing rates.
Liquid droplet generators have been used to atomize fluids having a low-viscosity fluids having a viscosity generally equal to that of water (.mu..sub.water =1.0 cP). Liquid droplet generators operate by passing a fluid through an orifice and providing a disturbance to the fluid at a certain frequency so that a stream of fluid discharged from the orifice will tend to separate into droplets. For example, liquid droplet generators have been employed in ink jet printer applications for atomizing a fluid jet into a stream of droplets having a diameter less than 1 mm. Such liquid droplet generators have a transducer driven at high frequencies to disturb the fluids, for example, at frequencies of about 50,000 Hertz (Hz).
U.S. Pat. No. 5,248,087 to Dressier (one of the inventors of the present application) discloses a liquid droplet generator for agricultural spraying, spray drying, and fuel injection. This liquid droplet generator device incorporates one or more pairs of piezoelectric transducers and a piston to apply high frequency perturbations on a fluid to atomize the fluid into droplets. In particular, the droplet generator is driven at frequencies of about 5,550 Hz, about 9,640 Hz, and about 9,780 Hz.
The above-noted liquid droplet generators, although suitable for there intended purpose in atomizing low-viscosity fluids with high frequency perturbations, are not suitable for atomizing high-viscosity fluids that require low frequency perturbations. In addition, in the above-noted liquid droplet generators, fluid is contained in a fixed size chamber or reservoir and the transducers are operable only at select natural frequencies of the transducer system, for example, at 7,000 Hz, 13,000 Hz, and 17,000 Hz, and within a range thereof of about 100 Hz. Such configurations limit the ability to optimize the performance of the liquid droplet generator.
Therefore, there is a need for an apparatus selectively operable at low frequencies for breaking up jets of high-viscosity fluid into monodisperse droplets. In addition, there is a need for an apparatus and method in which the performance of the apparatus can be optimized to minimize the power required for droplet formation.